1. Technical Field
The present invention generally relates to a method and apparatus of transmitting information in a television system and, more particularly, to a pulse generator including a memory for storing pulses for modulation onto a carrier or subcarrier of a composite television signal.
2. Description of the Prior Art
In conventional cable television systems, information may be transmitted from a headend location to individual terminal units in the system. This information provides a variety of information, for example, describing how a composite video signal is being scrambled, as well as other information useful to the terminal unit as is well-known in the art. Ways of transmitting such information include amplitude modulating the FM sound carrier, a subcarrier, or an out-of-band carrier signal with the information.
FIG. 1 illustrates adjacent NTSC television channels. Each television channel has a bandwidth of 6 MHz, within which a composite video and audio signal are transmitted. The video portion is transmitted as signals amplitude modulating a picture carrier and occupying 5.5 MHz of the channel, while the audio portion is transmitted in a 50 KHz band as a frequency modulated signal on a sound carrier 4.5 MHz above the picture carrier frequency. A color subcarrier is located 3.58 MHz above the video carrier. The sound carrier of the first channel is centered 0.25 MHz below the upper end of the second channel. It is important to ensure that when information is amplitude modulated onto the sound carrier of the first channel, interference with the second channel is not generated. The transmission of information by amplitude modulating a square wave pulse with its sharp transitions is desirable in terms of detection by a receiver. However, when the sound carrier of the first channel is amplitude modulated using the square wave pulses illustrated in FIG. 2A, the resulting spectrum 25 can be seen to interfere with the second channel. Wave shaping techniques may be used to reduce this interference. For example, the spectrum 30 generated by amplitude modulating the audio subcarrier of the first channel with the sine squared pulses illustrated in FIG. 2B does not significantly interfere with the second channel.
A circuit arrangement for amplitude modulating the sound carrier using the sine squared pulses of FIG. 2B is illustrated in FIG. 3A. The circuit arrangement includes a control circuit 40, a logic gate 45, a sine squared shaping filter 50, and an amplitude modulator 55. Control circuit 40 such as a microprocessor applies control signals to logic gate 45, such as a TTL gate, in accordance with the data to be transmitted. Logic gate 45 outputs square wave pulses in response to the control signals, one such pulse designated as 47. Square wave pulse 47 is supplied to sine squared shaping filter 50, such as a fourth order sine squared shaping filter, which shapes pulse 47 to a sine squared pulse 52. Sine squared pulse 52 has a sufficiently sharp transition to permit detection by receivers, but avoids the interference problems associated with rectangular pulses. Sine squared pulse 52 is supplied to audio modulator 55 which amplitude modulates the sine squared pulse onto an intermediate frequency sound carrier. The output of audio modulator 55 is supplied to a modulator (not shown) for generating a television signal suitable for transmission, for example, over a cable television system. A terminal unit in the cable television system receives the television signal and recovers the information amplitude modulated onto the sound carrier in order, for example, to descramble scrambled video information.
The amplitude modulation of pulses onto the sound carrier may also be used in an effort to defeat the ability of unauthorized or pirate terminal units from recovering the transmitted descrambling information to thereby obtain access to premium programming. Accordingly, pulses may be transmitted which unauthorized terminal units incorrectly detect as descrambling information, but which authorized terminal units ignore. This may be accomplished, for example, by transmitting pulses 40 and 42 as illustrated in FIG. 2C. Unauthorized terminal units may detect pulses 40 and 42 as timing pulses, thereby inhibiting the ability of the unauthorized terminal unit to recover the proper timing information necessary to descramble the scrambled picture. Authorized terminal units would ignore pulses 40 and 42. It will be appreciated that compatibility among various systems sensitive to data amplitude modulated on the sound carrier may be achieved if, for example, certain pulses modulated onto the sound carrier by a scrambler of a first system are not detected by receivers designed for use in a second system. Again referring to FIG. 2C, a scrambler designed to transmit data to a receiver of a first system may nonetheless be compatible with receivers of a second system if, for example, pulses 40 and 42 have an amplitude sufficient to be detected by receivers of the first system, but not by receivers of the second system.
Pulses such as those in FIG. 2C may be generated by the circuitry of FIG. 3B which includes control circuit 66, logic gate 67, pulse modification circuit 68, first and second sine squared shaping filters 70 and 72, timing control 75, gain adjustment circuit 80, and amplitude modulator 81. Control circuit 66 applies control signals to logic gate 67 in accordance with the data to be transmitted. Logic gate 67 outputs square wave pulses in response to the control signals. Pulse modification circuit 68 may be included to shift the level of the square wave since pulses of different amplitudes are to be generated. First and second sine squared shaping filters 70 and 72 shape the pulses. Gain adjusting circuitry 80 is utilized to adjust the gain of the signal in one path relative to the signal in the other path. Timing control 75 switches between the outputs of filters 70 and 72 at appropriate times. Amplitude modulator 81 modulates the pulses onto a sound carrier at an intermediate frequency. While such circuitry may be implemented, the timing control may be difficult to implement in practice and the sine squared filters must be specifically designed to defeat each different pirating technique or to achieve compatibility with each different system.
The amplitude modulation of signals on the sound carrier at the line rate of 15.734 KHz (f.sub.H) can also interfere with the second audio program (SAP) of a stereo transmission. The second audio program is modulated onto a subcarrier having a frequency five times the line rate f.sub.H. If a high quality demodulator is not utilized, AM to FM conversion of the energy modulated onto the audio subcarrier at the line rate f.sub.H can result in the false detection of a second audio program or, more likely, a distortion of the second audio program. This problem may be addressed, for example, by providing a notch filter in the receiver to remove the fifth harmonic of the signal modulating the sound carrier at the line rate. However, such an arrangement can be difficult to implement and further adds group delay to the filtered signal.